Thyroid hormone action: effect of triiodothyronine on mitochondrial adenine nucleotide translocase in vivo and in vitro.

Adenine nucleotide translocase (AdNT) levels were measured as the exchange of extramitochondrial against intramitochondrial adenosine diphosphate (ADP) in liver, spleen, and testes mitochondria isolated from normal and hypothyroid rats using the "back-exchange" and atractyloside-stop method of Pfaff and Klingenberg. The results provide confirmation of previous reports that mitochondria from hypothyroid rats show a markedly diminished AdNT activity, which is restored to normal levels within 72 hours by intraperitoneal injection of 10 to 20 micrograms triiodothyronine (T3)/100 g body weight. The latter dose was found in dose-response studies to result in maximal stimulation of AdNT in liver mitochondria. Qualitatively similar results on AdNT activity were obtained in liver mitochondria within 30 to 60 minutes following intravenous injection into hypothyroid rats of a more physiological dose of T3 (40 ng/100 g body weight). AdNT in mitochondria isolated from spleen and testes (organs that do not exhibit a calorigenic response after administration of thyroid hormone to the whole animal) failed to respond to thyroidectomy and to administration of T3. More recently, we have observed that in vitro replacement of T3 also stimulates AdNT activity in hypothyroid liver mitochondria. The enzyme adenosine triphosphate (ATP) synthase was examined as another possible candidate for direct hormonal stimulation of mitochondria. Simultaneous determinations on the same rats after intraperitoneal injection of T3 (20 micrograms/100 g body weight) showed little or no effect on ATP synthase until after 37 to 85 hours, whereas enhanced activity of the translocator was regularly observed at 17 hours.(ABSTRACT TRUNCATED AT 250 WORDS)